In a conventional dual-mode (heating and air conditioning) circuit, when the pressure of the refrigerant fluid in the condenser reaches the same value as the pressure of the fluid at the outlet of the compressor, the momentary discharges of fluid from the heating loop, intended to control pressure in the loop, are no longer effective to reduce the compressor outlet pressure. As a result, the pressures prevailing at the suction and delivery sides of the compressor will increase without it being possible to reduce them. This leads to a runaway situation such that the maximum safe pressures in the loop of the circuit are quickly reached.
Such a situation can be found in particular in a circuit of a heating and air conditioning apparatus for the cabin of a vehicle, of the general kind described in French patent specifications FR 2 717 126A and 2 731 952A. These circuits comprise a first branch containing an evaporator followed by the compressor, a second branch containing the condenser and a third branch containing no condenser. The second and third branches are arranged in parallel with each other so as to constitute, with the first branch, a cooling loop and a heating loop respectively. Switching means are provided for sending the fluid leaving the first branch selectively into the second branch or into the third branch. The heating and air conditioning apparatus further includes means for passing into the cabin air which has undergone heat exchange with the evaporator.
When the fluid is flowing in the cooling loop, the dual-mode circuit operates as a conventional air conditioning circuit. When the fluid is flowing in the heating loop, the evaporator is used as a heat exchanger for heating the cabin, with the fluid normally remaining in the gaseous state: this is the heating mode.
In a circuit of the above type, switching means are provided, typically comprising one or more discharge valves, for discharging fluid to the condenser from the heating loop. This enables the circuit to pass momentarily from the configuration of the heating loop to the configuration of the air conditioning loop so that the compressor can discharge fluid into the condenser, thereby reducing the mass of fluid in circulation once the configuration corresponding to the heating loop has been restored.
In such discharges, the pressure of the refrigerant fluid in the condenser can exceed safety limits, or other limits appropriate to correct operation in the loop.